1. Field of the Invention
The present invention relates to the structure of a repeatedly-usable recording medium, such as a semiconductor recording medium for example, in which the reproducible period of data which is recorded, i.e. the effective duration of record data, can be freely set and also relates to a control method thereof.
2. Description of the Related Art
By virtue of the development of microfabrication technique for semiconductor, high-density, high-integration semiconductor apparatuses have been manufactured and supplied. Especially, in a semiconductor memory prepared by a submicron rule, the number of memory elements formed in one chip has become enormous, and thus it has become possible to record long-running music and videos, coupled with the reduction of bit's unit price as well as the development of data compression technique.
In the semiconductor memory of this kind, which can be easily used as a recording medium even from a cost viewpoint, there may be expected the following utilization form in which recorded data is deleted or cannot be read out after a predetermined period: e.g. a commuter pass or ticket and a utilization in the rental field of renting music, a video soft, and the like.
As the semiconductor memory corresponding to this utilization form, for example, Japanese Patent Application Laying Open NO. Hei 10-189780 discloses one example of the technique. According to this disclosure, a gate insulating layer is provided with: a first layer of a silicon dioxide film; a second layer of a silicon nitride film; and a third layer of a silicon dioxide film, for example, and the composition ratio of silicon/nitrogen (Si/N) in the second layer of the silicon nitride film is set so as to correspond to a desired data effective duration. This construction is used to try to positively form a shallow charge lap, which causes the deterioration of memory holding, and reduce electrical charges accumulated in a charge accumulation mechanism to shorten the lifetime of the data holding, thereby setting the data effective duration. In the memory in this form, the lifetime of record data is determined by controlling the composition ratio of Si/N in the silicon nitride film in the chip production.
However, the above-described memory in which the composition ratio of Si/N is controlled at the time of production so as to determine the effective duration of record data has a problem that the memory lifetime is determined at the time of the production and cannot be changed. Moreover, the lifetime is based on the production time point, and when data is actually recorded, it is difficult to learn the elapsed time from the production time point, so that it is difficult or impossible to learn the effective duration of the recorded data from that time point, which leaves difficulties in practical use.